This invention deals with a satellite, preferably of low mass (typically below 500 kg or even below 100 kg depending on whether it is classified as a minisatellite or a microsatellite) for long distance laser ranging, that is the accurate measurement of the separation of two points far removed one from the other. To this end, it is known to install a transmitter-receiver laser unit at one of these points and a retroreflecting target at the other point: the distance measurement is derived from the time required for a laser pulse to travel in one direction, between the transmitter-receiver and the target.
As an example, one may have to measure distances of several hundreds (or even thousands) of kilometers with an accuracy of the order of a few centimeters. Typically, the distance between a point on the Earth and an object in space (a spacecraft or a natural satellite (moon) or an artificial satellite orbiting the Earth), or more generally between two space-borne bodies, such as a spacecraft orbiting another planet and the like, may need to be measured. By accumulating such measurements and using several targets, the distances for example between several points on the terrestrial globe may then be determined with accuracy using triangulation techniques or the like.
In practice, the retroreflecting target embodies one or several retroreflectors, which preferably have three mutually orthogonal and contiguous plane reflector faces, assembled so as to form a "cube corner", and the diagonal of an imaginary cube to which this cube corner belongs constitutes a reference axis referred to as the retroreflector normal. A property of such a retroreflector target is to reflect incident rays back parallel to themselves. Thus, reflecting the laser beam back towards the transmitter, even over very large distances, does not require the cube corner axis to be pointed towards the transmitter, provided that the laser pulse penetrates the cube corner, and of course, that the orthogonality of the reflecting surfaces is as perfect as possible.
In fact, the pulse reflected by a perfect cube corner has only one diffraction lobe, with an energy peak in the reflection direction and whose equivalent width can be given, as a first approximation, by the relation l/d, where l is the wavelength of the incident pulse and d is the average transverse dimension of the target (improperly called the effective diameter, or simply, the diameter). Thus, for a 0.5 .mu.m wavelength (green in color) and a target diameter of 10 cm, the lobe width (in the absence of any disturbing medium) is about one arcsecond (this width is in fact the subtended angle through which an observer placed at the target would see this reflected pulse).
As long as the subtended angle through which the target sees the separation between the transmitter (at the time when the incident pulse is transmitted) and the receiver (at the time when the reflected pulse reaches the latter) is smaller that the lobe width, the ranging principle indicated above may be appropriately employed. However, the received to transmitted energy ratio of the transmitter-receiver unit decreases whenever there is a large relative velocity between the transmitter-receiver unit and the retroreflecting target, transversal to the direction of a straight line joining them; in fact it is known in this case to define a velocity aberration angle, which depends on the ratio between the relative transverse velocity and the laser beam velocity (or speed of light). When this velocity aberration angle becomes greater than the lobe width, this means that the receiver is transversely offset from the diffraction lobe of the reflected pulse, when the latter reaches the location previously occupied by the transmitter-receiver unit at the instant of pulse emission.
To compensate for this velocity aberration, it has already been proposed to change by a few arcseconds the right angles between the three reflecting faces of the target, so as to widen the return beam. However, in practice, when this modification angle is increased from zero, the diffraction lobe, which was initially unique in a three dimensional graph correlating the energy density transmitted by the target in one direction with two tilt angles characterizing the spatial orientation of this direction relative to the incident pulse direction, widens and develops in its center a depression surrounded by an irregular ring; specifically, this ring embodies six peaks which interpenetrate one another and are arranged in a circle.
Within a given retroreflector, one thus obtains an "omnidirectional" correction of the velocity aberration, which however becomes insufficient when the latter substantially exceeds the average width of the six individual lobes, because any additional increase in these modification angles results in the breaking-up of the aforementioned ring into six separate lobes: the compensation effect becomes uncertain according to whether, as a result of the relative transverse velocity, the receiver intercepts one of these six lobes, the light energy is reduced since it does not exceed more than a sixth of the total energy.
It has thus been proposed to provide, on the target, a plurality of small retroreflectors, randomly oriented about their normals so as to generate an overall return pulse composed of a plurality of elementary pulses, the sets of six lobes of which would be mutually complementary, so as to form a ring-like lobe. However, the use of several retroreflectors contributing to this overall lobe deteriorates the measurement accuracy, in particular by virtue of the differences in the position of these retroreflectors on the target, which induce a time-wise spreading of the pulse arrivals at the receiver, and by virtue of the small size of these retroreflectors, which limits the energy of the individual return pulses to a low level.
The applicant has already proposed to mitigate these drawbacks, and even for large transverse velocities, to ensure an efficient correction of the velocity aberration and to obtain high accuracy measurements, while maintaining a high ratio of received to emitted light energy.
In order to do so the applicant has proposed, in U.S. Pat. No. 5,202,743, owned by the common assignee hereof, to abandon the symmetry principle applied so far in laser ranging, where the orientation of cube corners about their normals was of little importance, or where the same requirements applied to all three dihedral angles.
In this respect, the applicant taught how to generate a return pulse in a very small number of retroreflectors, designed and arranged so that the return energy is broken-up, no longer into six lobes, but into only two, which are parallel to the transverse relative velocity.
More specifically, in the aforementioned application, the applicant proposed a long distance laser ranging device embodying a transmitter/receiver unit, conventional per se, which was adapted to transmit and receive a laser pulse, and a retroreflecting target adapted to receive this laser pulse and send it back parallel to itself, in which the transmitter-receiver unit moved relative to the target along a trajectory, in some points of which the unit had an overall relative velocity V and, normal to an instantaneous imaginary line joining it to the target, had a transverse relative velocity V.sub.t which is on average of the order of at least 1 km/sec. The target embodies at least one retroreflector in the form of a cube corner whose field of view intercepts the trajectory in at least one portion thereof and which has three plane reflecting faces which determine three dihedral angles converging towards an apex, of which two faces are orthogonal to the third one while forming with each other an angle differing from 90.degree. by a deviation value e equal to at least one arcsecond. The faces converge into a so-called "corrected dihedral angle", contained in a plane intersecting the cube corner normal and at least approximately perpendicular to the average orientation of the relative transverse velocity in this or these portion(s) of the trajectory, with the deviation .epsilon. ranging between the minimum and maximum values of the expression V.sub.t /(C.sin.THETA.), in radians, over the portion(s) of trajectory intercepted by the retroreflector field of view, where V.sub.t and .THETA. are, for a given point in this or these portions, respectively the relative transverse velocity and the angle between the imaginary line joining this point to the target and the corrected dihedral angle, and where C is the speed of light.
In other words, when the trajectory of the transmitter-receiver relative to the target is of an orbital nature (that is when the transmitter-receiver (conversely the target) travels along an orbit with respect to a celestial body--a planet such as Earth or the moon--carrying the target (conversely the transmitter-receiver)), the plane P is at least approximately normal to the planes defined by the orbital trajectory portions intercepted by the cube corner field of view.
The transverse velocities known today usually correspond to deviation angles in the range of approximately 1 to 10 arcseconds.
Such a retroreflector has a maximum transverse dimension (called "effective diameter") typically of about 10 cm.
In fact, in this prior application, the applicant was particularly concerned with ground targets, whereas the transmitter-receiver was carried by a spacecraft.
The present application deals with the opposite scenario, in which a transmitter-receiver is placed on the ground, and one (or several) retroreflector(s) are space-borne.
In fact, laser ranging between a transmitting-receiving ground station and a geodetic satellite with conventional retroreflectors is now known to enable distance measurements with an accuracy of the order of 1 to 2 cm. This residual uncertainty depends for the moment mainly on the uncertainties arising from models of laser transmission through the Earth's atmosphere.
The future use of laser stations equipped with a two-color transmission capacity is expected, provided sufficient light energy is returned, to enable the uncertainty related to transmission through the atmosphere to be reduced to a few millimeters at the most. The main source of uncertainty will then be the geometry of the geodetic satellite itself.
In this regard, the existing geodetic satellites are simply dense spherical balls, several tens of centimeters in diameter, equipped with a large number of small, solid cube corner retroreflectors. A single laser pulse, emitted from the station and intercepting the sphere, thus results (as mentioned above) in a plurality of return pulses which are not quite simultaneous, and having therebetween random phase shifts. When received at the ground, this results in a return pulse which is weakly coherent and time spread, giving rise to an uncertainty in the identification of the exact instant of the pulse return.